This paper seeks to refocus the attention of environmentalists on the importance of population trends to environmental sustainability and identifies prevention of unintended pregnancy as potential common ground for environmentalists and family planning advocates. The health and other welfare benefits of preventing unintended pregnancy are felt most keenly by individual women, men, and their families. At the same time, however, preventing unwanted pregnancies usually results in smaller family size, an important factor in slowing population growth and, as a result, a source of broader benefits—including environmental benefits—to society.